Thermally compensated current sensing of intrinsic power converter elements

ABSTRACT

A DC-to-DC converter includes one or more power switches, a pulse width modulation circuit for generating control pulses for the power switches, and an output inductor connected to the power switches. A thermally compensated current sensor is connected to an intrinsic current sensing element exhibiting a temperature-based parameter non-linearity. The thermally compensated current sensor has a temperature coefficient that substantially matches a temperature coefficient of an intrinsic power converter element used to measure current flow, thus linearizing the current measurement. Also, a current feedback loop circuit cooperates with the pulse width modulation circuit to control the power switches responsive to the thermally compensated current sensor.

RELATED APPLICATION

[0001] This application is based upon prior filed copending provisionalapplication No. 60/313,986 filed Aug. 21, 2001, the entire disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of electroniccircuits, and more particularly, to DC-DC converters and associatedmethods.

BACKGROUND OF THE INVENTION

[0003] Typically, DC-DC converters use current flow information toprovide value added functions and features. For example, limiting thecurrent during an overload is commonly implemented as a safety feature.Such a current limit feature would use a signal proportional to outputcurrent limiting level. A resistor inserted between the output and theload could generate the desired signal. However, the resistance of thissensor is the subject of a trade-off between power dissipation andsignal amplitude. Typically, the signal level at current limit isapproximately 0.1 volt, to be well above the noise floor. The sensingresistor's power dissipation is proportional to the load current at thelimit level. At high current levels, the power dissipation can beexcessive.

[0004] Eliminating the sensing resistor improves the DC-DC converter'sefficiency. Instead of an additional resistive element, current flow ismeasured using the intrinsic elements within the power convertercomponents. For example, U.S. Pat. No. 5,982,160 to Walters et al. andentitled “DC-to-DC converter with inductor current sensing and relatedmethods” teaches that the current flow information in an inductor can bereconstructed as a voltage across a resistor-capacitor network. Thismethod uses the intrinsic resistance of the inductor's winding as thecurrent sensing element.

[0005] Another method to eliminate the current sensing resistor measuresthe voltage dropped across the nearly constant, on-state resistance ofone of the switching MOSFETs in the converter. The method samples thevoltage drop during the conduction interval of the MOSFET to reconstructthe current flow information. Both of these methods make use of thefundamental power converter components as current sensing elements andthey avoid using a dissipative element in the power path.

[0006] The intrinsic current sensing methods in the above examples canonly approximate the actual current flow. These methods suffer inaccuracy when compared with the current sensing resistor. For example,utilizing the inductor's winding resistance as the current sensingelement suffers both an initial tolerance error and a variation withtemperature. An inductor's winding initial resistance varies with thelength and diameter of the winding's wire, as well as the specificmanufacturing procedure. This same wire resistance increases as afunction of temperature. Therefore, the reconstructed voltage signal isa function of the inductor windings' mechanical tolerance andtemperature as well as the current flow.

SUMMARY OF THE INVENTION

[0007] In view of the foregoing background, it is therefore an object ofthe invention to provide low power dissipation while accuratelymeasuring and processing current information with thermal compensationin a switching DC-to-DC converter.

[0008] This and other objects, features and advantages in accordancewith the present invention are provided by a DC-to-DC converterincluding one or more power switches, a pulse width modulation circuitfor generating control pulses for the power switches, and an outputinductor connected between the power switches and an output terminal. Athermally compensated current sensor is connected to the output inductorfor sensing current in the output inductor. The thermally compensatedcurrent sensor has a temperature coefficient that substantially matchesa temperature coefficient of the output inductor. Also, a currentfeedback loop circuit cooperates with the pulse width modulation circuitto control the power switches responsive to the thermally compensatedcurrent sensor.

[0009] The power switches preferably include a low side field effecttransistor and a high side field effect transistor connected together.The thermally compensated current sensor may be connected in parallelwith the output inductor and may comprise a resistor and a capacitorconnected in series. The resistor of the thermally compensated currentsensor may be a positive temperature coefficient resistor.

[0010] Alternatively, the thermally compensated current sensor may beconnected to the at least one power switch for providing a sensedcurrent proportional to a current being conducted through the outputinductor. Here, the thermally compensated current sensor has atemperature coefficient that substantially matches a temperaturecoefficient of an on-state resistance of the power switches. Also, inthis embodiment, the thermally compensated current sensor is connectedbetween the power switches and the current feedback loop circuit, andcomprises a positive temperature coefficient resistor.

[0011] Another aspect of the present invention relates to a multiphaseDC-to-DC converter having multiple channels. Each of the channelsincludes a power device with, e.g. a low side power switch and a highside power switch connected together. A pulse width modulation circuitgenerates control pulses for the power device, and an output inductor isconnected between the power device and the output terminal. A thermallycompensated current sensor is connected to the power device in eachchannel for providing a sensed current proportional to a current beingconducted through the output inductor. The thermally compensated currentsensor has a temperature coefficient that substantially matches atemperature coefficient of an on-state resistance of the low side powerswitch. Also, a current feedback loop circuit cooperates with the pulsewidth modulation circuit for controlling the power device responsive tothe thermally compensated current sensor.

[0012] In an alternative embodiment of the multiphase DC-to-DCconverter, instead of the thermally compensated current sensor, afeedback resistive network is connected between an input of the controlcircuit of each of channels and the output terminal. The feedbackresistive network includes a negative temperature coefficient resistorhaving a temperature coefficient that substantially matches atemperature coefficient of an on-state resistance of the monitored powerswitch of the power devices.

[0013] A method aspect of the present invention is directed toregulating a DC-to-DC converter comprising an output terminal, powerswitches, a pulse width modulation circuit for generating control pulsesfor the power switches, an output inductor connected between the powerswitches and the output terminal, and a current feedback loop circuitcooperating with the pulse width modulation circuit for controlling thepower switches. The method includes sensing current passing through theinductor using a thermally compensated current sensor connected to theoutput inductor. Again, the thermally compensated current sensor has atemperature coefficient that substantially matches a temperaturecoefficient of the output inductor. Furthermore, the current feedbackloop circuit operates to control the at least one power switch inresponse to the thermally compensated current sensor.

[0014] Alternatively, the method may include providing a sensed currentproportional to a current being conducted through the output inductorusing a thermally compensated current sensor connected to at least onepower switch. Here, the thermally compensated current sensor has atemperature coefficient that substantially matches a temperaturecoefficient of an on-state resistance of the at least one power switch.The current feedback loop circuit controls the at least one power switchin response to the thermally compensated current sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic diagram of a DC-to-DC converter of thepresent invention.

[0016]FIG. 2 is a schematic diagram of a second embodiment of a DC-to-DCconverter of the present invention.

[0017]FIG. 3 is a graph illustrating the load line characteristics of aconventional DC-to-DC converter without thermal compensation.

[0018]FIG. 4 is a graph illustrating the load line characteristics ofthe DC-to-DC converter of FIG. 2.

[0019]FIG. 5 is a schematic diagram of a multiphase DC-to-DC converterin accordance with the present invention.

[0020]FIG. 6 is a schematic diagram of an alternative embodiment of themultiphase DC-to-DC converter of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention will now be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0022] Some functions and features of the DC-DC converter do not requirehigh precision current flow information and the intrinsic methodsdiscussed above offer an efficiency advantage. However, other functionsand features demand better precision. An example demanding highprecision is load line characteristic or droop feature found in DC-DCpower conversion for microprocessors. Here, the output voltage isprogrammed to decrease (droop) with increasing load current. The droopfeature positions the output voltage at the optimum level prior to aload transient so the transient voltage excursion stays withinacceptable levels with the minimum output capacitance. The precision isrequired to minimize not only the bulk output capacitance but also thehigh-frequency distribution capacitance.

[0023] Turning now to FIG. 1 of the drawings, the DC-to-DC converter 10in accordance with a first embodiment of the present invention is nowdescribed. The DC-to-DC converter 10 provides a controlled voltage Voutto a load 22. In the illustrated embodiment, the DC-to-DC converter 10includes a pair of power switches, a high side switch 12, and a low sideswitch 14 connected to a source voltage Vin. Of course, as will bereadily appreciated by those skilled in the art, the DC-to-DC converter10 in other embodiments may include only the high side switch 12, with adiode substituted in the position of the low side switch 14. Inaddition, although MOSFET transistors are illustrated, othersemiconductor switches may be used as will also be understood by thoseskilled in the art.

[0024] The DC-to-DC converter 10 also includes the schematicallyillustrated pulse width modulation circuit 16 which would alsopreferably include switch drivers. This circuit portion is more fullydescribed, for example, in U.S. Pat. No. 5,717,322 to Hawkes et al. andU.S. Pat. No. 5,793,193 to Hodgins, both assigned to the assignee of thepresent invention. Both of these patents are also incorporated byreference herein in their entirety. The pulse width modulation circuit16 generates a series of pulse width modulated control pulses for thepower switches 12, 14 to regulate the output voltage Vout coupled to theload 22. Those of skill in the art will readily appreciate theconstruction and operation of the pulse width modulation circuit 16without further detailed discussion.

[0025] The illustrated DC-to-DC converter 10 also includes an outputinductor 18 coupled between the load 22 and a node between the high andlow side switches 12, 14. A diode may also be connected between groundand the node between the high and low side power switches 12, 14. Anoutput capacitor 20 is connected in parallel across the load 22 as willalso be readily appreciated by those skilled in the art.

[0026] This embodiment of the present invention provides a currentsensor 30 connected in parallel with the output inductor 18 for sensingcurrent passing through the inductor. The current sensor 30 preferablycomprises a resistor Rsen and a capacitor Csen connected together inseries. The current flow information in the inductor 18 can bereconstructed as a voltage across the resistor-capacitor network. Thismethod uses the intrinsic resistance of the inductor's winding as thecurrent sensing element. As would be appreciated by the skilled artisan,the intrinsic resistance of the inductor's winding has a temperaturecoefficient. The current sensor 30 is connected to the illustratedfeedback signal processing loop circuit 32 cooperating with the pulsewidth modulation circuit 16 for controlling the power switches 12, 14responsive to the current sensor. The resistor Rsen and capacitor Csenmay have respective values so that the current sensor 30 is asubstantially instantaneous current sensor.

[0027] Importantly, the current sensor 30 is a thermally compensatedcurrent sensor having a temperature coefficient that substantiallymatches a temperature coefficient of the output inductor 18. The loadline accuracy can be improved by compensating for the intrinsic currentsensing temperature characteristic. In other words, the current loopgain is modified as a function of temperature to cancel the temperaturecharacteristic of the intrinsic current sensing element, e.g. the outputinductor 18. Accordingly, the disadvantages of the prior art inductorcurrent sensing techniques are overcome.

[0028] The feedback signal processing circuit 32 may include a voltageregulation loop circuit cooperating with a peak current control loopcircuit for setting a peak current level. The current sense signal isprocessed through the schematically illustrated feedback signalprocessing loop circuit 32 to properly condition the signal for pulsewidth modulation circuit 16. Those of skill in the art will readilyappreciate the construction of the many possible and equivalentvariations of the feedback signal processing loop circuit 32, such asdisclosed, for example, in the above cited U.S. Pat. Nos. 5,717,322 and5,793,193.

[0029] The DC-to-DC converter may also include an overload detectioncircuit which uses the sensed current signal from the current sensor 30to prevent overloads as will be readily understood by those skilled inthe art. The DC-to-DC converter 10 may also include additionalfeatures/circuit portions not shown for clarity including, for example,soft start and slope compensation circuit portions. The DC-to-DCconverter 10 may also include a hysteretic comparator, not shown, forswitching between a normal operating mode and a discontinuous lowcurrent demand mode.

[0030] Another embodiment of the invention will now be described whilereferring to FIGS. 2-4. Here, the DC-to-DC converter 10′ includes athermally compensated current sensor 30′ connected to the power switches12, 14 for providing a sensed current proportional to a current beingconducted through the output inductor 18. Here, the thermallycompensated current sensor 30′ has a temperature coefficient thatsubstantially matches a temperature coefficient of an on-stateresistance of one of the power switches 12, 14, e.g. the low side powerswitch 14. Also, in this embodiment, the thermally compensated currentsensor 30′ is connected between the power switches 12, 14 and thecurrent feedback circuit 32, and includes a positive temperaturecoefficient resistor.

[0031] As discussed above and in U.S. Pat. No. 6,246,220, the Rds(on)method samples the voltage drop during the conduction interval of theMOSFET to reconstruct the current flow information. For this example,the voltage is sampled across the lower MOSFET 14 (shown in FIG. 2)using the resistor Rsen connected to feedback signal processing 32including virtual-ground. The sensed current (Isen) is proportional tothe inductor current (Il) by the following relationship:Isen=Il×(Rds/Rsen). As the MOSFET temperature increases, its Rds(on)increases which causes a corresponding increase in the sensed currentIsen. The Isen signal is further processed within the system to providea load line characteristic that is also a function of MOSFETtemperature.

[0032] The Isen signal can be thermally compensated by selecting a Rsenresistor with the appropriate thermal characteristics. For example,selecting a positive temperature coefficient (PTC) resistor that matchesthe MOSFETs Rds(on) temperature coefficient minimizes the Isen, anddroop voltage, dependency on MOSFET temperature.

[0033]FIG. 3 shows a typical load line specification and the RSStolerance analysis of a conventional DC-to-DC converter utilizing powerswitch's, e.g. a MOSFET's, on-state resistance (Rds(on)) as the currentsensing element. The analysis includes the variations due to thereference, voltage setting resistors, and MOSFET parameters. The largestvariation is due to the temperature characteristic of the MOSFET.

[0034]FIG. 4 illustrates the RSS tolerance analysis of a DC-to-DCconverter with thermal compensation in accordance with the presentinvention. The minimum and maximum load lines fall within thespecification.

[0035] Another aspect of the present invention relates to a multiphaseDC-to-DC converter 40 having first and second channels, and which willbe described with reference to FIGS. 5 and 6. Each of the channelsincludes a power device with, e.g. a low side power switch 14 and a highside power switch 12 connected together. A pulse width modulationcircuit 16 generates control pulses for the power device, and an outputinductor 18 is connected between the power device and the outputterminal. For multiphase power converters a PTC resistor is required oneach power channel. Thus, a thermally compensated current sensor Rsen1,Rsen2 is connected to the power device in each channel for providing asensed current proportional to a current being conducted through therespective output inductor. The thermally compensated current sensorRsen1, Rsen2 has a temperature coefficient that substantially matches atemperature coefficient of an on-state resistance of the low side powerswitch 14.

[0036] In an alternative embodiment, the multiphase DC-to-DC converter40′ (FIG. 6), includes a feedback resistive network connected between aninput of the pulse width modulation circuit or control circuit of eachof channels and the output terminal. The feedback resistive network Rfbincludes a negative temperature coefficient resistor Rntc having atemperature coefficient that substantially matches a temperaturecoefficient of an on-state resistance of the low side power switch 14 ofthe power devices.

[0037] This approach compensates for the thermal effects of currentsensing utilizing a negative temperature coefficient resistor Rntc. Theembodiment uses a single NTC device for temperature correction inmultiphase converters as compared with the PTC compensation method. Theresistor Rfb in the embodiment of FIG. 5 is replaced with an NTCresistor network to provide correction of the Isen signal. NTC resistorstypically have non-linear thermal characteristics. The resistance can belinearized over the temperatures of interest using a network of standardresistors 42 connected as shown in FIG. 6.

[0038] Another aspect of the invention relates to a method forregulating a DC-to-DC converter 10, 10′ of the type as described aboveand comprising power switches 12, 14, a pulse width modulation circuit16 for generating control pulses for the power switches, an outputinductor 18, and a feedback signal processing circuit 32 cooperatingwith the pulse width modulation circuit. The method preferably includessensing current passing through the inductor 18 using a thermallycompensated current sensor 30 connected in parallel with the outputinductor. Again, the thermally compensated current sensor has atemperature coefficient that substantially matches a temperaturecoefficient of the output inductor. Furthermore, the current feedbackloop circuit 32 operates to control the power switches 12, 14 inresponse to the thermally compensated current sensor. The current sensor30 preferably comprises a resistor Rsen and a capacitor Csen connectedtogether in series.

[0039] Alternatively, the method may include providing a sensed currentproportional to a current being conducted through the output inductor 18using a thermally compensated current sensor 30′ (FIG. 2) connected tothe power switches. Here, the thermally compensated current sensor 30′has a temperature coefficient that substantially matches a temperaturecoefficient of an on-state resistance of one power switch. The currentfeedback loop circuit 32 controls the at least one power switch inresponse to the thermally compensated current sensor 30′.

[0040] It is understood by those skilled in the art that all the abovedescribed embodiments can be applied to the inductor wire currentsensing approach or the Rds(on) current sensing approach.

[0041] Many modifications and other embodiments of the invention willcome to the mind of one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is understood that the invention is not to belimited to the specific embodiments disclosed, and that modificationsand embodiments are intended to be included within the scope of theappended claims.

That which is claimed is:
 1. A DC-to-DC converter comprising: at leastone power switch; a pulse width modulation circuit for generatingcontrol pulses for the at least one power switch; an output inductorconnected to the at least one power switch; a thermally compensatedcurrent sensor connected to the output inductor for sensing current inthe output inductor, the thermally compensated current sensor having atemperature coefficient that substantially matches a temperaturecoefficient of the output inductor; and a current feedback loop circuitcooperating with the pulse width modulation circuit for controlling theat least one power switch responsive to the thermally compensatedcurrent sensor.
 2. A DC-to-DC converter according to claim 1 wherein theat least one power switch comprises at least one field effecttransistor.
 3. A DC-to-DC converter according to claim 1 wherein the atleast one power switch comprises a low side field effect transistor anda high side field effect transistor connected together.
 4. A DC-to-DCconverter according to claim 1 wherein the at least one power switchcomprises a low side power switch and a high side power switch connectedtogether.
 5. A DC-to-DC converter according to claim 1 wherein thethermally compensated current sensor is connected in parallel with theoutput conductor and comprises a resistor and a capacitor connected inseries.
 6. A DC-to-DC converter according to claim 5 wherein theresistor of the thermally compensated current sensor comprises apositive temperature coefficient resistor.
 7. A DC-to-DC convertercomprising: at least one power switch; a pulse width modulation circuitfor generating control pulses for the at least one power switch; anoutput inductor connected to the at least one power switch; a thermallycompensated current sensor connected to the at least one power switchfor providing a sensed current related to a current being conductedthrough the output inductor, the thermally compensated current sensorhaving a temperature coefficient that substantially matches atemperature coefficient of an on-state resistance of the at least onepower switch; a current feedback loop circuit cooperating with the pulsewidth modulation circuit for controlling the at least one power switchresponsive to the thermally compensated current sensor.
 8. A DC-to-DCconverter according to claim 7 wherein the at least one power switchcomprises at least one field effect transistor.
 9. A DC-to-DC converteraccording to claim 7 wherein the at least one power switch comprises alow side field effect transistor and a high side field effect transistorconnected together.
 10. A DC-to-DC converter according to claim 7wherein the at least one power switch comprises a low side power switchand a high side power switch connected together.
 11. A DC-to-DCconverter according to claim 7 wherein the thermally compensated currentsensor is connected between the at least one power switch and thecurrent feedback loop circuit, and the thermally compensated currentsensor comprises a resistor.
 12. A DC-to-DC converter according to claim11 wherein the resistor of the thermally compensated current sensorcomprises a positive temperature coefficient resistor.
 13. A multiphaseDC-to-DC converter comprising: at least first and second channels eachcomprising a power device including a low side power switch and a highside power switch connected together, a pulse width modulation circuitfor generating control pulses for the power device; an output inductorconnected to the power device, a thermally compensated current sensorconnected to the power device for providing a sensed current related toa current being conducted through the output inductor, the thermallycompensated current sensor having a temperature coefficient thatsubstantially matches a temperature coefficient of an on-stateresistance of the low side power switch, a current feedback loop circuitcooperating with the pulse width modulation circuit for controlling thepower device responsive to the thermally compensated current sensor. 14.A multiphase DC-to-DC converter according to claim 13 wherein each ofthe power switches comprises a field effect transistor.
 15. A multiphaseDC-to-DC converter according to claim 13 wherein the thermallycompensated current sensor is connected between the power device and thecurrent feedback loop circuit, and the thermally compensated currentsensor comprises a resistor.
 16. A multiphase DC-to-DC converteraccording to claim 15 wherein the resistor of the thermally compensatedcurrent sensor comprises a positive temperature coefficient resistor.17. A multiphase DC-to-DC converter comprising: at least first andsecond channels each comprising a power device including a low sidepower switch and a high side power switch connected together, a pulsewidth modulation circuit for generating control pulses for the powerdevice; an output inductor connected to the power device, a currentsensor connected to the power device for providing a sensed currentproportional to a current being conducted through the output inductor, acurrent feedback loop circuit cooperating with the pulse widthmodulation circuit for controlling the power device responsive to thecurrent sensor; and a feedback resistive network connected between aninput of the pulse width modulation circuit of each of the at leastfirst and second channels and the output terminal, and comprising anegative temperature coefficient resistor having a temperaturecoefficient that substantially matches a temperature coefficient of anon-state resistance of the low side power switch of the power device ofthe at least first and second channels.
 18. A multiphase DC-to-DCconverter according to claim 17 wherein each of the power switchescomprises a field effect transistor.
 19. A method of regulating aDC-to-DC converter comprising at least one power switch, a pulse widthmodulation circuit for generating control pulses for the at least onepower switch, an output inductor connected to the at least one powerswitch, and a current feedback loop circuit cooperating with the pulsewidth modulation circuit for controlling the at least one power switch,the method comprising: sensing current passing through the inductorusing a thermally compensated current sensor connected to the outputinductor, the thermally compensated current sensor having a temperaturecoefficient that substantially matches a temperature coefficient of theoutput inductor; and operating the current feedback loop circuit tocontrol the at least one power switch in response to the thermallycompensated current sensor.
 20. A method according to claim 19 whereinthe at least one power switch comprises at least one field effecttransistor.
 21. A method according to claim 19 wherein the at least onepower switch comprises a low side field effect transistor and a highside field effect transistor connected together.
 22. A method accordingto claim 19 wherein the thermally compensated current sensor isconnected in parallel with the output conductor and comprises a resistorand a capacitor connected in series.
 23. A method according to claim 22wherein the resistor of the thermally compensated current sensorcomprises a positive temperature coefficient resistor.
 24. A method ofregulating a DC-to-DC converter comprising at least one power switch, apulse width modulation circuit for generating control pulses for the atleast one power switch, an output inductor connected to the at least onepower switch, and a current feedback loop circuit cooperating with thepulse width modulation circuit for controlling the at least one powerswitch, the method comprising: providing a sensed current related to acurrent being conducted through the output inductor using a thermallycompensated current sensor connected to the at least one power switch,the thermally compensated current sensor having a temperaturecoefficient that substantially matches a temperature coefficient of anon-state resistance of the at least one power switch; and operating thecurrent feedback loop circuit to control the at least one power switchin response to the thermally compensated current sensor.
 25. A methodaccording to claim 24 wherein the at least one power switch comprises atleast one field effect transistor.
 26. A method according to claim 24wherein the at least one power switch comprises a low side field effecttransistor and a high side field effect transistor connected together.27. A method according to claim 24 wherein the thermally compensatedcurrent sensor is connected between the at least one power switch andthe current feedback loop circuit, and the thermally compensated currentsensor comprises a resistor.
 28. A method according to claim 27 whereinthe resistor of the thermally compensated current sensor comprises apositive temperature coefficient resistor.